In thermal oil recovery processes, channeling and override of injected fluids such as steam are common problems. Channeling can be caused by high permeability zones (thief zones) which can exist in the reservoir or, it can be caused by the high mobility ratio of the displacing fluid to the displaced fluid. The override effect occurs because of the density differences between the injected fluid and the oil. When either of these phenomena occurs, the injected fluid tends to breakthrough early at the production wells and bypass a considerable volume of the oil in the reservoir thereby resulting in inefficient oil recovery.
U.S. Pat. No. 4,074,757 to Felber et al. disclose a method of selective plugging of undesirable high permeability streaks and channels in oil bearing reservoirs during high temperature (250.degree. F. or higher) injection recovery processes such as steam flooding, underground combustion flooding or a naturally occurring high temperature reservoir, or the like. Improved sweep efficiency is obtained by injecting a gel-forming solution consisting essentially of sodium or ammonium lignosulfonate and water or brine in the absence of other gelation promoters and then allowing the high temperatures of the underground formation to promote gelation. To ensure that the injected gel-forming solution is confined largely to the high permeability zone of the formation, it is necessary to isolate the offending, high-permeability zone by one or more packers or plug back procedures. However, this method does not ensure that there is no tendency for the gel-forming solution to flow into the low permeability zones of the formation. Other U.S. patents teaching the use of polymers that are injected as monomer solutions and polymerize in-situ to effect plugging of highly permeable zones include U.S. Pat. No. 4,637,467 to Shaw et al. and U.S. Pat. No. 4,461,351 to Falk. These processes are not selective because the monomers before they polymerize can penetrate into high as well as the low permeability zones of the formation and packers are required to inject the monomer into selected portions of the formation.
The present invention provides a method by which the high-permeable thermal fluid swept zones of a formation are selectively plugged by temperature selective gel-forming solutions without plugging the less permeable unswept portion of the formation thereby allowing further thermal fluid injection to sweep the less permeable unswept portion of the formation resulting in more complete displacement of oil from the formation.